1. Field of the Invention
The present invention relates to an electric cooking apparatus such as an electric range or an electric oven.
2. Description of the Related Art
The conventional high frequency heating apparatus, as shown in FIG. 12, is composed of a rectifier circuit comprising a rectifier 2 for rectifying electric power supplied from a commercial power source 1, an inductor 3 and a capacitor 4; an inverter circuit comprising a capacitor 5, a transistor 6, a diode 7 and a boosting transformer 8; a high voltage rectifying circuit comprising a high voltage capacitor 9 and high voltage diodes 10 and 11; a magnetron 12 for generating a high frequency radio wave in response to the output from the high voltage rectifying circuit; a control circuit 13 for controlling the operating frequency of the transistor 6; and heating control section 15 for controlling a relay 14 and issuing a heating command to the control circuit 13.
FIGS. 13(A), (B) and (C) show a terminal voltage Vac across the capacitor 4 which is an input voltage to the inverter circuit, a collector-emitter voltage Vce of the transistor 6 and its envelope, and the envelope of the anode current Ia of the magnetron, respectively. The actual Ia waveform has the envelope as shown and the high frequency ripple similar to Vce. As indicated by broken lines, the magnetron 12 oscillates only during the period while Ia flows and Vce exceeds the voltage corresponding to threshold voltage of the magnetron 12. Specifically, in order to prevent reduction in the input power factor at the input terminal (i.e., an AC terminal of the rectifier 2) of a high frequency heating apparatus, the terminal across the capacitor 4 is obliged to have the voltage waveform as shown in FIG. 13(A). Thus, the magnetron 12 will perform an intermittent operation as shown in FIG. 13(C). On the other hand, the transistor 6 operates also during the period while Ia does not flow. This period does not almost contribute to the high frequency oscillation of the magnetron 12. This means that the transistor 6 operates wastefully.
The commercial power outlet in general homes is usually rated at 15A, and the indoor power wiring in the homes is also usually rated at 20A. For this reason, the maximum consuming current of the high frequency heating apparatus such as an electric range must be set for 13-14 A. This is because although the indoor wiring current capacity is 20 A, equipments such as a rice cooker and a toaster which are used simultaneously with the high frequency heating apparatus have usually the consuming current of 6-7 A.
Thus, in the conventional high frequency heating apparatus, as shown in FIG. 13(C), the oscillation period of the magnetron 12 is about 1/2 of the entire operation period, and also the input current is limited by the commercial power source. Hence, the instrument rated at e.g., AC 100 V usually has the input power limited to 1.3-1.4 kW. As a result, the high frequency output of the conventional heating apparatus is usually limited to at most about 600 W. The permissible maximum output rating for a short time is 700 W, and only the high frequency heating apparatus with such a radio wave has been put into practice.
Japanese Patent Publication No. 56- 21237 discloses the high frequency heating apparatus as shown in FIG. 14. In this high frequency heating apparatus, the AC voltage from a commercial power source 1 is rectified by a rectifier 16 to be converted into a DC voltage, and also a battery 17 is provided to supply the voltage to a DC--DC boosting circuit 20 using switches 18 and 19. Thus, electric power is supplied to a magnetron 12 through an interrupting circuit 21. This high frequency heating apparatus intends to provide radio wave output of 600 W (an input of 1200 W) to homes with small contract electric power. This is realized in such a way that energy is stored in the battery 17 with about 100 V while the apparatus is not used, and when the apparatus is used, the stored energy is taken out to drive the magnetron 12.
Although the above prior art wa directed to the electric range, it is also applied to a cooker using a heater such as an electric oven. In short, in the prior art, because of limitation due to the maximum consuming current from the commercial power source, the power source of 100 V permitted the cooking apparatus to be put into practice only with input power of 1.4 kW or less.
As described above, the prior art can only provide the radio wave output limited by the input current from a commercial power source; it is difficult to promote the feature of high speed heating (cooking) of the high frequency heating apparatus. Further, since the conventional arrangement of FIG. 14 provided with a battery must have capability of storing very large energy, the apparatus has disadvantages that it is large-scaled and expensive, and also requires to switch the output from the high-voltage large electric power battery. It can be also assumed that both the DC voltage from the rectifier (half-wave rectifier) 16 and the output voltage from the battery 17 are simultaneously supplied to the DC--DC boosting circuit 20. However, only taking out both voltages gives rise to inconveniences including considerable reduction in the input power factor in the commercial power source 1 and occurrence of an unstable operation due to mis-matching of internal impedances of both power sources. Thus, this cannot be actually realized. As a result, the arrangement of FIG. 14 also suffers from the limitation due to the input current from the external power source such as a commercial power source, and so cannot solve the problem that the AC input voltage of e.g. 100 V limits the radio wave output to about 600 W-700 W.
Accordingly, it is very difficult to provide a high frequency heating apparatus or cooking cooker such as an electric oven which can realize high speed heating (cooking) with a large high-frequency output not limited by the current capacity of the external power source and also has a compact and light-weight structure. A cooking apparatus suited to modern food life requiring high speed cooking has been demanded.